Vehicle headlamp systems have experienced many adaptations over the years, including the use of incandescent halogen lamps which result in increased light output and lower energy consumption. Unlike traditional incandescent lamps, where a filament is surrounded by an inert gas such as argon (Ar), incandescent halogen lamps envelop the filament with a gas composition that includes a gas from the halogen group. In both designs, the filament, generally tungsten (W), is supported by and connected to electric current carrying lead wires which supply the filament with current and cause it to become a glowing “white hot” according to a process commonly known as incandescence. A consequence of the incandescence process is that the filament is heated to extreme temperatures and begins to evaporate such that tungsten atoms are released into the surrounding volume. In traditional incandescent lamps, the released tungsten atoms are deposited onto a large glass bulb surrounding the filament, thereby darkening the bulb and weakening the filament. Unique to incandescent halogen lamps is the ability for the evaporated atoms to combine with the surrounding halogen gas and subsequently redeposit themselves back onto the filament, a process sometimes referred to as the halogen cycle. In this process, when the evaporated tungsten atoms are in the vicinity of a surrounding quartz envelope, they are somewhat cooled and combine with the halogen gas to form a tungsten halide molecule. This molecule then migrates back to the vicinity of the heated filament, which decomposes the molecule such that the tungsten is deposited back onto the filament and the halogen gas is released into the surrounding volume. Thus, the incandescent halogen lamp undergoes a type of recycling process, thereby increasing the life of the lamp. Moreover, the incandescent halogen lamp can be operated at a hotter temperature, thereby increasing the light emission per unit of energy. While incandescent halogen lamps improve many of the characteristics of vehicle headlamp systems, there still remains much room for further improvement.
For instance, a portion of the total light emitted from incandescent halogen lamps often reflects off of interior components of the lamp, such as the lead wires, and results in uncontrolled stray light appearing as glare to oncoming drivers. U.S. Pat. No. 4,302,698 issued Nov. 24, 1981 to Kiesel et al. discloses an incandescent halogen lamp for use in a vehicle headlamp assembly. The embodiment shown in FIG. 3b discloses two filaments that are supported by three current carrying lead wires. Two of the lead wires connect to the filaments at their lower most ends, and therefore do not significantly interfere with light emitted from the filaments. The third lead wire, however, connects with both filaments at their uppermost ends and consequently extends alongside the filaments. Halogen lamp assemblies having lead wires generally positioned alongside of the filaments have the potential to reflect stray light off of the lead wires which appears as glare to oncoming drivers. Thus, it would be advantageous to design an incandescent halogen lamp assembly where the lead wires do not significantly interfere with the light emitted from the filaments.
Addressing this concern, some designs have incorporated filaments having long leg portions and short lead wires, as will be subsequently discussed. In these designs, the filament has a long, thin leg portion that extends from its uppermost end and bends downward at approximately a 90° angle. The thin leg portion extends alongside the filament until it connects with a thicker lead wire proximate the lowermost end of the filament. Because the filament leg is substantially thinner than the shortened lead wire, it does not interfere with the light emission to the extent that a thicker lead wire running alongside the filament would. Accordingly, designs of this nature realize the benefits of utilizing an incandescent halogen lamp and reduce the amount of stray light, and hence glare, attributable to reflection off of internal lamp components. While these designs can improve the illumination performance of the lamp assembly, they can also compromise its structural integrity. The thin filament leg portion is weaker than the substantially thicker and stronger lead wires previously discussed. Consequently, these designs may have difficulty satisfying testing requirements, particularly vibrational testing.
Thus, it would be advantageous to provide an incandescent halogen lamp design that reduces glare due to reflection from interior lamp components, such as lead wires, but does not compromise the structural integrity of the lamp.